And we all know Hulk would not be the main villain (he wouldn't be the villain at all even against the Avengers because they are the ones who betrayed him) and would eventually rejoin the Avengers and help take down the real villain. It's not a terrible idea at all and creates good conflict and drama, things every moviegoer loves.

And if the Avengers are the ones who "betrayed" him, then *they're* the bad guys.

Again: either (a) the Hulk or (b) one or more of the other Avengers comes off looking like a villain and/or a douche. That's not "good conflict and drama," that's a disservice to the characters and the fans.

And we've already seen Hulk vs. Thor and Hulk vs. Black Widow; you get Hulk back into the ring against the Avengers again, and it's going to be been-there/done-that all over again ("that's soooooo 2012").

It's a terrible idea.

Planet Hulk: not a terrible idea.
World War Hulk, in any shape or form: a terrible idea. In the comics, and in the movies.